Phenotype of proliferating (BrdU⁺) CD4 and CD8 T cells in various tissues of normal neonatal macaques. Memory cells are defined as CD95⁺ (A,C), and activated cells as CD69⁺ (B,D). Data are expressed as means ± SE. Note that the majority of the proliferating CD4⁺ and CD8⁺ T cells in the intestine have a memory phenotype (CD95⁺) whereas fewer memory cells are proliferating in other tissues in early development. Note approximately 90% of the proliferating CD4⁺ T cells in the jejunum of 3-day-old infants are have whereas only approximately 55% are proliferating in blood through the first 14 days of development (A). Fewer proliferating T cells have a memory phenotype in lymph nodes and spleen, yet these rapidly increase with age. In addition, most of the proliferating T cells in the intestine are “activated” (CD69⁺), whereas proliferating T cells rarely coexpress CD69⁺ in the spleen, and none in blood. Interestingly, the spleen has higher levels of activated, proliferating cells than blood or lymph node, but still fewer than the intestine.